1. Field of the Invention
This invention relates to an interferometer, and more particularly, to an interferometer having features suited for the inspection of moving surfaces.
2. Background Information
A surface profile interferometer is a device for determining the roughness of a surface or the height of a step change in the thickness of a part being measured. Such a step change may be caused, for example, by the application of a metal film to a substrate in the manufacture of a printed circuit board or an integrated microcircuit. In general terms, an interferometer is an optical instrument in which two beams of light derived from the same monochromatic source are directed along optical paths of different length, in which the difference in length determines the nature of an interference pattern produced when the light beams are allowed to interfere. Since the beams of light are derived from the same monochromatic source, they are identical in wavelength. At equal path distances from the source, they are also in phase with one another. Phase differences between the beams therefore result only from differences in path length.
The phenomenon of light wave interference results from the mutual effect of two or more waves passing through the same region at the same time, producing reinforcement at some points and neutralization at other points, according to the principle of superposition.
With a photoelectric shearing interferometer, the height of a step change in a test surface may be measured using polarized light passed through a slit, through a Wolloston prism, and through a microscope objective lens, to form two images of the slit, with one image on each side of the step change. The beams reflected by the test surface pass through the lens and the prism, with an image being formed by two orthoganally polarized beams. The phase difference between these beams, which is determined by the height of the step, may be measured by the linear movement of a weak lens in a lateral direction (transverse to the beam) until the phase difference is exactly cancelled, as determined by the use of an electro-optic modulator, an analyzer, a photomultiplier, and a phase-sensitive detector, which are used together to detect the phase equality of the two interfering beams. The accuracy of the system depends on the precision to which the linear movement of the weak lens can be measured. Thus, a difference in phase between two orthogonal polarizations is measured, with the beams laterally displaced by the Wollaston prism, so that the system is not a common-path interferometer.
The Wollaston prism makes use of the phenomenon of double refraction or birefringence, through which a crystal of a transparent anisotropic material refracts orthogonally polarized light beams at different angles. Crystals such as calcite, quartz, and mica exhibit this property. A Wollaston prism includes two wedge-shaped segments held together with adjacent polished surfaces extending along a plane at an oblique angle to the optical axis of the device. The outer surfaces of the Wollaston prism lie along planes perpendicular to the optical axis of the device. The two segments of the Wollaston prism are composed of a birefringent material, with the crystal axes of the material lying perpendicular to each other and to the optical axis of the device.
For example, if a beam of light consisting of two sub-beams polarized orthogonally to each other is directed along the optical axis of the device to a Wollaston prism, the two beams will not be refracted at the initial surface of the prism, since it lies perpendicular to the direction of both beams. However, when the two beams reach the oblique surfaces inner surfaces of the two segments of the prism, refraction will occur, with the two beams being refracted at different angles because of the birefringence of the material of which the prism segments are composed. When the two beams reach the opposite external side of the prism, they are again refracted.
While the above discussion describes a Wolloston prism comprising two wedges of birefringent material, it is possible and often advantageous to form a prism of this kind using three or more such wedges, joined at two or more oblique planes. When this is done, the outer surfaces of the prism remain perpendicular to the optical center of the device.
Thus, a number of methods have been developed for using interferometers to provide accurate measurements of very small surface features. However, since these methods are based on rather elaborate and painstaking processes in which a very small surface area is held in place to be viewed through an interferometer, they are difficult to apply to the materials of a mass production process making, in large volumes, pads which would benefit from inspection by means of interferometry. What is needed, for example, is a way to apply a scanning process allowing a relatively large test surface to be checked without stopping for the measurement of individual areas. Such a process could then be applied, for example, to disks used for data storage.